In the preparation of adiponitrile, an important intermediate in nylon production, 1,3-butadiene is initially reacted with hydrogen cyanide in the presence of nickel(0) which is stabilized with phosphorus ligands to give pentenenitriles. In addition to the main products of the hydrocyanation, 3-pentenenitrile and 2-methyl-3-butenenitrile, numerous secondary components are also obtained. Examples thereof are 2-pentenenitriles, 2-methyl-2-butenenitriles. C9-nitriles and methylglutaronitrile. 2-methyl-3-butenenitrile is formed in significant amounts. Depending on the catalyst used, the molar ratio of 2-methyl-3-butenenitrile formed to 3-pentenenitrile may be up to 2:1.
In a second hydrocyanation, 3-pentenenitrile is subsequently reacted with hydrogen cyanide to give adiponitrile over the same nickel catalyst with addition of a Lewis acid. For the second hydrocyanation, it is essential that the 3-pentenenitrile is substantially from of 2-methyl-3-butenenitrile. A hydrocyanation of 2-methyl-3-butenenitrile would lead to methylglutaronitrile which constitutes an undesired by-product. Accordingly, in an economic process for preparing adiponitrile, there has to be a separation of 3-pentenenitrile and 2-methyl-3-butenenitrile.
In order to likewise be able to utilize 2-methyl-3-butenenitrile for the preparation of adiponitrile, processes have been proposed for isomerizing 2-methyl-3-butenenitrile to linear pentenenitrile, especially 3-pentenenitrile.
For instance, U.S. Pat. No. 3,676,481 describes the discontinuous, batchwise isomerization of 2-methyl-3-butenenitrile in the presence of Ni(0), a phosphite ligand and certain Lewis acids. After the isomerization, the resulting product mixture is distilled off from the catalyst system. A disadvantage in this process is that of the high residence times during the isomerization, the high thermal stress on the thermally sensitive catalyst during the isomerization and during the subsequent distillation. The high thermal stress on the catalyst leads to undesired degradation of the catalyst.
The German patent application DE 103 11 119.0 to BASF AG, which has an earlier priority date but was unpublished at the priority date of the present application, describes a process for isomerizing 2-methyl-3-butenenitrile to linear pentenenitrile in the presence of a system comprising Ni(0) catalysts and Lewis acids. In this case, a mixture comprising 2-methyl-3-butenenitrile and linear pentenenitrile is withdrawn distillatively from the reaction mixture during the isomerization. A disadvantage in this process is that the product stream withdrawn still contains distinct amounts of unconverted 2-methyl-3-butenenitrile.
It is common to all known processes for isomerizing 2-methyl-3-butenenitrile that 2-methyl-3-butenenitrile cannot be fully converted to 3-pentenenitrile owing to the position of the thermodynamic equilibrium. Unconverted 2-methyl-3-butenenitrile has to be fed to the isomerization step for economic performance of the process. However, in the isomerization of 2-methyl-3-butenenitrile, (Z)-2-methyl-2-butenenitrile is obtained as a by-product and would accumulate in the cycle stream in the case of recycling of 2-methyl-3-butenenitrile, since, in the course of the removal of 3-pentenenitrile from the isomerization product stream by distillation, it would distill over together with the 2-methyl-3-butenenitrile owing to the very similar vapor pressures.
U.S. Pat. No. 3,865,865 describes the removal of 2-methyl-2-butenenitrile from a mixture with 2-methyl-3-butenenitrile. The removal is carried out by treating the mixture of the nitriles with an aqueous solution which consists of sulfite and bisulfite ions. This forms the bisulfite adduct of 2-methyl-2-butenenitrile which transfers to the aqueous phase. The resulting organic phase is depleted to 50% of the original content of 2-methyl-2-butenenitrile. The process of U.S. Pat. No. 3,865,865 is laborious, since a phase separation of an organic from an aqueous phase is required. Furthermore, this separation can only be integrated with difficulty into an overall process for preparing adiponitrile. An additional disadvantage in this process is that the resulting organic phase first has to be fully freed of water before further use in hydrocyanation reactions using nickel(0) catalysts with phosphorus(III) ligands, since the phosphorus(III) ligands are otherwise irreversibly hydrolyzed and thus inactivated. Another disadvantage in this process is that the resulting bisulfite adducts, for the purpose of reuse of the conjugated nitriles, as described in U.S. Pat. No. 3,865,865, can only be dissociated under drastic conditions and only with moderate yield.